1. Field of the Invention
The present invention relates to a direct-heated flow measuring apparatus having a film resistor which serves as a temperature detecting means as well as an electric heater. Such a direct-heated flow measuring apparatus can be used, for example, for measuring the flow rate of engine intake air.
2. Description of the Related Art
Generally, in an internal combustion engine, the amount of intake air is one of the most important parameters for controlling the fuel injection amount, ignition timing, and the like. A flow measuring apparatus, i.e., an airflow meter, is provided for measuring the same. One of the more common prior art airflow meters is the vane-type, but this is disadvantageous in scale, response speed characteristics, and the like, and therefore, airflow meters having temperature-dependent resistors have been developed, in which these disadvantages of scale, response speed characteristics, and the like are avoided (see: U.S. Pat. No. 3,975,951).
There are two types of airflow meters having temperature-dependent resistors, i.e., the heater-type and direct-heated type. The heater-type airflow meter may consist of an electric heater resistor provided in an intake-air passage of an engine and two temperature-dependent resistors arranged on the upstream and downstream sides of the electric heater resistor (see: U.S. Pat. No. 3,957,951). In this case, the temperature-dependent resistor on the downstream side is used for detecting the temperature of air heated by the heater resistor, while the temperature-dependent resistor on the upstream side is used for detecting the temperature of non-heated air. The current flowing through the heater resistor is controlled to provide a constant difference in temperature between the two temperature-dependent resistors, and thus the mass flow rate of air is determined by detecting the voltage applied to the heater resistor.
In this heater-type airflow meter, if an upstream temperature-dependent resistor is not provided and the current of the heater resistor is controlled to provide a constant temperature of the downstream temperature-dependent resistor, the voltage applied to the heater resistor is detected as representing the volume flow rate of air.
On the other hand, the direct-heated type airflow meter may consist of a film resistor (i.e., a sensing element) which serves not only as an electric heater, but also as a temperature-detecting means for detecting the temperature of the heated air (U.S. Pat. Nos. 3,747,577 and 4,279,146). Also, the direct-heated type airflow meter may consist of a temperature-dependent resistor for detecting the temperature of non-heated air. Thus, the current flowing through the film resistor is controlled to provide a constant difference in temperature between the film resistor and the temperature dependent resistor, thereby detecting the voltage applied to the film resistor as representing the mass flow rate of air. In this direct-heated type airflow meter, too, if a temperature-dependent resistor is not provided and the current of the heater resistor is controlled to provide a constant temperature of the film resistor, the voltage applied to the film resistor is detected as representing the volume flow rate of air.
Since the film resistor of the direct-heated type airflow meter serves as a temperature-detecting means for heated air, that is, an additional temperature detecting means for heated air is not necessary, the direct-heated type airflow meter is smaller in size than the heater-type airflow meter.
In the direct-heated type airflow meter, the film resistor may consist of an insulating substrate such as a ceramic substrate or monocrystalline silicon substrate, a resistance layer of platinum (Pt), gold (Au), etc. on the insulating substrate, and a heat-resistant resin on the resistance pattern. Here, it is impossible to burn off suspended particles (mainly, carbon particles) adhered to the boundary region and the stagnation region of the film resistor, since a temperature higher than 800.degree. C. is required for such burn-off and the heat-resistant resin has a low melting temperature such as 400.degree. C. Therefore, the suspended particles remain adhered to the film resistor, increasing the heat capacity and reducing the heat dissipation characteristics thereof, and therefore, reducing the sensitivity and response speed characteristics of the airflow meter. Also, where a heat-resistant resin is not provided for covering the film resistor, which is, in this case, made of heat-resistant platinum, when such a burn-off is often carried out, the film resistor is exposed to a temperature higher than 800.degree. C., so that the resistance characteristics of the film resistor are changed, thus inviting a drift in the output of the air-flow meter. In this case, it is impossible to accurately carry out a flow measurement.
Further, when a heat-resistant platinum layer is formed on a ceramic substrate or a semiconductor substrate, and such a burn-off is carried out, distortions occur between the platinum layer and the substrate, thus changing the resistance characteristics of the platinum layer.
Further, to carry out a burn-off, an additional burn-off circuit is provided, thus increasing the cost of the airflow meter (see U.S. Pat. No. 4,196,622).